What is the difference between a Mannequin and a Womennequin?
According Terraria Wiki, the difference is that it will show your gear in their female sprites. So if your character is female and you want to show exactly how does your gear look, you need the Womannequin.
Gear appears slightly differently when equipped by male or female characters. The mannequin displays armor exactly as if a male character were wearing it, while a womannequin displays armor exactly as if a female character were wearing it.